Underground drilling involves drilling a bore through a formation deep in the Earth by connecting a drill bit to a drill string. During rotary drilling, the drill bit is rotated by a top drive or other rotary drive means at the surface, where a quill and/or other mechanical means connects and transfers torque between the rotary drive means and the drill string. During drilling, the drill bit is rotated by a drilling motor mounted in the drill string proximate the drill bit, and the drill string may or may not also be rotated by the rotary drive means.
Drilling operations can be conducted on a vertical, horizontal, or directional basis. Vertical drilling refers to drilling in which the trajectory of the drill string is inclined at less than about 10° relative to vertical. Horizontal drilling refers to drilling in which the drill string trajectory is inclined about 90° from vertical. Directional drilling refers to drilling in which the trajectory of the drill string is being deliberately controlled to maintain the wellbore on the planned course. Correction runs generally refer to wells that have deviated unintentionally and must be steered or directionally drilled back to the planned course.
Various systems and techniques can be used to perform vertical, directional, and horizontal drilling. For example, steerable systems use a drilling motor with a bent housing incorporated into the bottom-hole assembly (BHA) of the drill string. A steerable system can be operated in a sliding mode in which the drill string is not rotated and the drill bit is rotated exclusively by the drilling motor. The bent housing steers the drill bit in the desired direction as the drill string slides through the bore, thereby effectuating directional drilling. Alternatively, the steerable system can be operated in a rotating mode in which the drill string is rotated while the drilling motor is running.
Rotary steerable tools can also be used to perform directional drilling. One particular type of rotary steerable tool can include pads or arms located on the drill string adjacent the drill bit and extending or retracting at some fixed orientation during some or all revolutions of the drill string. Contact the between the arms and the surface of the wellbore exerts a lateral force on the drill string adjacent the drill bit, which pushes or points the drill bit in the desired direction of drilling.
Directional drilling can also be accomplished using rotary steerable motors which include a drilling motor that forms part of the BHA, as well as some type of steering means, such as the extendable and retractable arms discussed above. In contrast to steerable systems, rotary steerable motors permit directional drilling to be conducted while the drill string is rotating. As the drill string rotates, frictional forces are reduced and more bit weight is typically available for drilling. Hence, a rotary steerable motor can usually achieve a higher rate of penetration during directional drilling relative to a steerable system, since more of the combined torque and power of the drill string rotation and the downhole motor are available to be applied to the bit, because of the friction reduction in the wellbore induced by the constant rotation.
Directional drilling requires real-time knowledge of the angular orientation of a fixed reference point on the circumference of the drill string in relation to a reference point on the wellbore. The wellbore reference point is typically magnetic north in a vertical well, or the high side of the bore in an inclined well. This orientation of the drillstring reference point relative to the fixed reference point is typically referred to as toolface. For example, drilling with a steerable motor requires knowledge of the toolface so that the pads can be extended and retracted when the drill string is in a particular angular position, so as to urge the drill bit in the desired direction.
When based on a reference point corresponding to magnetic north, toolface is commonly referred to as magnetic toolface (MTF). When based on a reference point corresponding to the high side of the bore, toolface is commonly referred to as gravity tool face (GTF). GTF is usually determined based on measurements of the transverse components of the local gravitational field, i.e., the components of the local gravitational field perpendicular to the axis of the drill string, which are typically acquired using an accelerometer and/or other sensing device included with the BHA. MTF is usually determined based on measurements of the transverse components of the Earth's local magnetic field, which are typically acquired using a magnetometer and/or other sensing device included with the BHA.
Obtaining, monitoring, and adjusting the drilling direction conventionally requires that the human operator must manually scribe a line or somehow otherwise mark the drill string at the surface to monitor its orientation relative to the downhole tool orientation. That is, although the GTF or MTF can be determined at certain time intervals, the top drive or rotary table orientation is not known automatically. Consequently, the relationship between toolface and the quill position can only be estimated by the human operator. It is known that this relationship is substantially affected by reactive torque acting on the drill string and bit. Consequently, there has been a long-felt need to more accurately gauge the relationship between toolface and quill position so that, for example, directional drilling can be more accurate and efficient.